Oscillators are typically utilized as an electronic circuit for generating periodic analog signals. Generally, oscillators may be classified into two categories. One category of oscillator is termed a harmonic oscillator, which is configured to generate a sinusoidal output voltage based on self-exciting oscillation. The other category of oscillator is termed a relaxation oscillator. The relaxation oscillator is employed to generate a non-sinusoidal output voltage, such as a square-wave output voltage, a sawtooth-wave output voltage, or a triangular-wave output voltage. A conventional relaxation oscillator includes non-linear elements such as transistors for periodically releasing energy stored in capacitors or inductors, and thereby instantaneously modulating the waveform of the output signal. On the other hand, due to the flourishing development of smart phones and tablet computers, touch panels have been extensively employed as an interface for user to input signals to the computer host. The contemporary touch panel usually adopts the technique of relaxation oscillation to sense the frequency variation of the voltage generated across the capacitors located on the touch point of the touch panel, and the sensed results are further provided for back-end processors to perform analytical procedures.
In recent years, a plethora of technical research reports in connection with relaxation oscillator have been published. The publications conveying such technical research reports include U.S. Pat. No. 7,375,535 issued to Kutz et al., U.S. Pat. No. 7,307,485 issued to Snyder et al., U.S. Pat. No. 8,159,462 issued to Seguine, and U.S. Pat. No. 8,058,937 issued to Qin et al. Nonetheless, the relaxation oscillator architecture proposed in these references inevitably requires a field-effect transistor (FET) switch to be disposed on the output node of the oscillator, in which the signal input end of the FET switch is connected to a current source used for charging/discharging a capacitor, and the signal output end of the FET switch is connected to a capacitor used for generating a periodic oscillating signal. Hence, the relaxation oscillator disclosed in the aforementioned references unavoidably suffers from the clock feedthrough problems. The clock feedthrough effect would impose a voltage offset on the output signal of the FET switch, which would in turn affect the operation accuracy of the oscillator.
In addition, Chen et al. disclose another relaxation oscillator architecture in their U.S. Pat. No. 8,373,502. The relaxation oscillator taught by this reference includes an operational transconductance amplifier and an operational amplifier, and additionally includes six switches to selectively switch between a pair of reference voltages to be inputted to both input terminals of the operational transconductance amplifier and the non-inverting input terminal of the operational amplifier. In this reference, the output current signal of the operational transconductance amplifier is configured to charge/discharge a capacitor for generating a periodic oscillating signal, and the voltage signal generated across the capacitor by the output current signal is inputted to the inverting input terminal of the operational amplifier. A pair of output clock signals of the operational amplifier are complementary to each other and are fed to the control terminals of the six switches for controlling their switching operations. Although this reference avoids the deficiency of clock feedthrough by forbidding the disposal of switches between the current source used for charging/discharging the capacitor and the capacitor used for generating a periodic oscillating signal, this reference requires six switches to select the desired reference voltages. Thus, a large portion of the circuit board area will be occupied and the manufacturing cost will be heightened accordingly. What is worse, the noise induced in the circuit will be aggravated.
There is an inclination to develop a relaxation oscillator that can address the problem of clock feedthrough with a minimum number of switch elements.